There is known an apparatus that processes an image captured by an onboard camera for fog determination (e.g., patent document 1). The fog determination method in patent document 1 is based on the degree of image blurredness. Since a blurred image results from a fog, the method estimates the degree of image blurredness for fog determination. To estimate the degree of image blurredness, the method first performs differential processing (or differential calculation) for each pixel on an entire image to calculate the edge amount of brightness. Based on the edge amount, the method estimates the degree of image blurredness. Since a blurred image indicates a small edge amount as a whole, it is possible to estimate the degree of image blurredness based on the edge amount.
The apparatus according to patent document 1 uses a result of the fog determination for the white road line recognition. When an image captured by the onboard camera is used for the white road line recognition, a fog blurs the image and makes the white road line recognition difficult. Accordingly, the fog determination is performed before the white road line recognition. Patent document 1 also describes that a fog lamp turns on when the fog is determined to occur.
The apparatus according to patent document 2 determines not only fog, but also image blurredness. The reason follows. When the low visibility due to fog and the like or a dirt on a windshield blurs the image, it becomes difficult to correctly monitor outside the vehicle (e.g., to monitor a distance between a subject vehicle and an object outside).
According to patent document 2, the determination method for image blurredness calculates a brightness edge for an image in a fixed area at the top center of an image. The degree of image blurredness is determined based on a ratio at which the edge exceeds a specified value.                Patent Document 1: JP-3444192 B2 (JP-H11-326541 A)        Patent Document 2: JP-3312729 B2 (JP-2001-28746 A)        
In patent documents 1 and 2, the fog determination is performed by determining whether or not an image captured by the onboard camera is blurred. As described in patent document 1, calculating the edge amount for the entire image increases an image processing load. As described in patent document 2, an edge is calculated only in the fixed area at the center top of the image. An image processing load is smaller than that in patent document 1. When an object is close to the subject vehicle, however, the object is relatively clearly captured even in fog. When an object close to the subject vehicle is captured in the fixed area, the fog determination becomes inaccurate. There are some cases where an object close to the subject vehicle is captured in the fixed area at the top center of the image. For example, a forward vehicle is running on the same lane with a small vehicle-to-vehicle distance. In addition, when the vehicle is running on a relatively sharp curve, the fixed area captures a relatively closely located scene at a roadside.